Multiple drug resistance (MDR) mediated by the human mdr-1 gene product was initially recognized during the course of developing regimens for cancer chemotherapy (Fojo et al., 1987, Journal of Clinical Oncology 5:1922-1927). A multiple drug resistant cancer cell line exhibits resistance to high levels of a large variety of cytotoxic compounds. Frequently these cytotoxic compounds will have no common structural features nor will they interact with a common target within the cell. Resistance to these cytotoxic agents is mediated by an outward directed, ATP-dependent pump encoded by the mdr-1 gene. By this mechanism, toxic levels of a particular cytotoxic compound are not allowed to accumulate within the cell.
MDR-like genes have been identified in a number of divergent organisms including numerous bacterial species, the fruit fly Drosophila melanogaster, Plasmodium falciparum, the yeast Saccharomyces cerevisiae, Caenorhabditis elegans, Leishmania donovanii, marine sponges, the plant Arabidopsis thaliana, as well as Homo sapiens. Extensive searches have revealed several classes of compounds that are able to reverse the MDR phenotype of multiple drug resistant human cancer cell lines rendering them susceptible to the effects of cytotoxic compounds. These compounds, referred to herein as "MDR inhibitors", include for example, calcium channel blockers, anti-arrhythmics, antihypertensives, antibiotics, antihistamines, immuno-suppressants, steroid hormones, modified steroids, lipophilic cations, diterpenes, detergents, antidepressants, and antipsychotics (Gottesman and Pastan, 1993, Annual Review of Biochemistry 62:385-427). Clinical application of human MDR inhibitors to cancer chemotherapy has become an area of intensive focus for research.
On another front, the discovery and development of antifungal compounds for specific fungal species has also met with some degree of success. Candida species represent the majority of fungal infections, and screens for new antifungal compounds have been designed to discover anti-Candida compounds. During development of antifungal agents, activity has generally been optimized based on activity against C. albicans. As a consequence, these anti-Candida compounds frequently do not possess clinically significant activity against other fungal species. However, it is interesting to note that at higher concentrations some anti-Candida compounds are able to kill other fungal species. This suggests that the antifungal target(s) of these anti-Candida compounds is present in these fungal species as well. Such results indicate that some fungal species possess a natural mechanism of resistance that permits them to survive in clinically relevant concentrations of antifungal compounds. Such a general mechanism of resistance to antifungal compounds in the fungi has remained undescribed.